"My investigation suggests the possibility that crimes against humanity may have been committed," Zeid Ra'ad Al Hussein said at the opening of the 36th session of the Human Rights Council, calling for an international probe.

Venezuela's crisis has caused food and medicine shortages, deadly unrest and calls for President Nicolas Maduro to quit.

Clashes with security forces at anti-government protests left 125 people dead from April to July.

"There is a very real danger that tensions will further escalate, with the government crushing democratic institutions and critical voices," Zeid warned.

He said an investigation by his office had noted the widespread use of "criminal proceedings against opposition leaders, recourse to arbitrary detentions, excessive use of force and ill-treatment of detainees, which in some cases amounts to torture."

Late last month, Zeid echoed international concerns that Venezuela was slipping into dictatorship, cautioning that democracy in the country was "barely alive, if still alive."

His office has previously criticised Venezuela's all-powerful constituent assembly and its "truth commission", which has been tasked with investigating several opposition leaders for treason.

On Monday, Zeid said he supported the concept of a truth commission, but stressed that "the current mechanism is inadequate."

"I therefore urge that it be reconfigured with the support and involvement of the international community," he said.

He urged the UN rights council "to establish an international investigation into the human rights violations in Venezuela."

Zeid also pointed out that Venezuela currently holds one of the 47 rotating seats on the Human Rights Council, and thus has a particular duty to "uphold the highest standards in the promotion and protection of human rights."

Without naming Venezuela specifically, he also called on the council to consider "the need to exclude from this body states involved in the most egregious violations of human rights."